There is an increasing demand to reduce the emissions produced by gas turbine combustors for aerospace, marine and industrial applications. One approach is to use lean premixed pre-vaporised (LPP) combustion in which liquid fuel is mixed and evaporated within a premixing duct. A typical LLP fuel injection apparatus is disclosed in EP0660038. The fuel-air mixture then flows into the combustion chamber where it is burnt. Low levels of oxides of nitrogen (NOx) emissions are obtained because the premixer produces a uniformly mixed fuel-air mixture at an equivalence ratio less than stoichiometric. This mixture burns at a relatively low flame temperature avoiding the NOx producing high temperature volumes of more conventional combustion systems.
To assist mixing, many premixing ducts incorporate a prefilmer mounted within the duct. This is usually disposed between radially adjacent swirl vanes. Fuel is shed from the downstream edge of the prefilmer, and is atomised as it passes through a shear region formed by the swirl vanes. In this way, fuel is always distributed from the centre of the duct and the chance of poor mixing due to over or under fuel penetration is avoided. In a typical LPP fuel injector this is the only purpose of the prefilmer.
Although LPP combustion systems can produce NOx emissions levels significantly lower than conventional combustion systems, there are severe disadvantages. One of these is combustion instability. Where variations in heat release and pressure are in phase, the magnitude of both fluctuations will increase. The severity of the combustion instability produced varies from an irritating noise to a force powerful enough to stall gas turbine compressors and cause structural damage to combustion systems. In a conventional aerospace gas turbine combustion system, different areas within the combustor operate at different air-fuel ratios. Here, fluctuations in heat release become out of phase relative to each other resulting in a reduction of the net heat-release. In an LPP system, as the system runs at a uniform air-fuel ratio, all parts of the combustion system tend to oscillate in phase with each other. Net heat release fluctuations therefore tend to be high.